The present invention relates to propulsion of a vehicle by means of a gas turbine.
The efficiency of the energy generation in a turbine power plant is a function of the pressure ratio of the compression process (including the ram pressure induced in a high-speed vehicle by decelerating the ambient air to bring it aboard the vehicle), the maximum temperature of the cycle, the efficiency of compression and expansion of the turbomachinery components, and the losses associated with pressure drops in static components, leakages, and parasitic air losses.
In reaction propulsion the efficiency of the propulsion process is also a strong function of the velocity at which the propulsive exhaust jet exits from the engine. The propulsive efficiency P.sub.e is given approximately by the equation: EQU P.sub.e =2/(1+a)
where a is the ratio of exhaust velocity (relative to the vehicle) to the vehicle's air speed. Maximum propulsive efficiency is achieved when the exhaust velocity is equal to the air speed or a=1. This ideal condition can be approached but only at the expense of making the propulsor very large and heavy, since the thrust F.sub.n per unit airflow W.sub.a is found to be approximately by the equation: EQU F.sub.n /W.sub.a =V.sub.o (a-1)
implying that values of a near unity give vanishingly small thrust per unit airflow. This equation indicated that the thrust per unit airflow is proportional to flight speed, so that a relatively large-mass flow must be handled at low speed.
In the design of a turbine engine for reaction propulsion, this balance of considerations is handled by a spectrum of propulsion systems. For very low flight speeds, very large propulsors are used to accelerate large amounds of air through small velocity increments, only slightly above that low flight speed, as typified by a turboprop aircraft. High speed propulsion systems are designed to accelerate smaller amounts of air to much larger velocities, as typified by the pure jet engine. Turbofans constitute intermediate devices in this propulsion spectrum.
Conventional takeoff and landing aircraft use turboprops for low subsonic flight speeds up to Mach number 0.7, high-bypass turbofans for propulsion from Mach number 0.6 through transonic flight speeds, and low-bypass turbofans or jet engines for propulsion at supersonic flight speeds or for mixed subsonic and supersonic aircraft. Low-bypass turbofans and turbojets are often provided with afterburners for thrust augmentation in special flight regimes.
Compared to the turbojet and, to a lesser degree, to the turbofan, the turboprop offers lower fuel consumption and a higher takeoff thrust. It has low engine noise level, and its propeller can be reversed to shorten the landing run. For these reasons the turboprop is an excellent power plant for aircraft in which these qualities are important. Because of propeller characteristics, the turboprop usually reaches peak operating efficiency at lower cruise speeds than the turbofan and is, therefore, better suited for transports in the speed range below 450 mi/hr, although it is basically possible to reach high subsonic and even supersonic flight speeds with a turboprop. At high altitudes, the turboprop achieves lower cruise fuel consumption than the best reciprocating engines of fuel per equivalent shaft horsepower.
The turbofan is an air-breathing aircraft gas turbine engine with operational characteristics between those of the turboprop and turbojet. Like the turboprop, the turbofan consists of a compressor-combustor-turbine unit, called a core or gas generator, and a power turbine. This power turbine drives a low-or medium-pressure-ratio compressor, called a fan, some or most of whose discharge bypasses the core.
The gas generator produces useful energy in the form of hot gas under pressure. Part of this energy is converted by the power turbine and the fan it drives into increased pressure of the fan airflow. This airflow is accelerated to ambient pressure through a fan jet nozzle and is thereby converted into kinetic energy. The residual core energy is converted into kinetic energy by being accelerated to ambient pressure through a separate core jet nozzle. The reaction in the turbomachinery in producing both atreams produces useful thrust.
In a turbojet, air approaches the inlet diffuser at a relative velocity equal to the flight speed. In passing through the diffuser the velocity of the air is decreased and its pressure increased. The air pressure is increased further as it passes through the compressor. In the combustion chamber a steady stream of fuel is injected into the air and combustion takes place continuously. The high-pressure hot gas passes through the turbine nozzles, which direct it at high velocity against the buckets on the turbine wheel, thereby causing the wheel to rotate. The turbine wheel drives the compressor to which it is connected through a shaft. This is the sole function of the turbine.
After the hot gas leaves the turbine, it is still at a high temperature and at a pressure considerably above atmospheric. The hot gas is discharged rearward through the exhause of the engine at a high velocity.
The thrust obtained is equal to the overall increase in momentum of the gas as a result of its passage through the engine. This thrust is given by the equation: EQU f=M(V.sub.j -V.sub.o)
where M=the mass flow of gas per secone through the engine, V.sub.j is the exhaust jet velocity, and V.sub.o is the airplane velocity.
Thus, each type of engine has an optimum flight regime, depending on airspeed, range, and altitude, in which its performance is superior to other propulsion systems. For low speeds and low altitudes, the turboprop engine gives the best performance; next comes the turbofan, and then the turbojet without afterburner. Until the advent of the present invention, no one turbine driven engine could fulfill the requirements met by the three separate engines described above.